The present invention relates to a golf ball on the surface of which many of the dimples formed are of non-circular shape, which golf ball has an excellent flight performance.
For a golf ball which has been hit to travel a long distance, it is important that the ball itself have a high rebound and that air resistance during flight be reduced by dimples arranged on the surface of the ball. A variety of methods for arranging dimples uniformly and to the highest possible density on the surface of the ball have been proposed in order to reduce the air resistance.
Here, the commonly employed dimple shape is a recess that is circular as seen from directly above. Because such circular dimples are used, attempts to arrange circular dimples to a high density, such as by making the width of the land between two neighboring circular dimples as close to zero as possible, result in the formation of triangular or quadrangular lands of a given size in areas surrounded by three or four of the arranged dimples. At the same time, it is essential to arrange the dimples as uniformly as possible on the spherical surface of the ball. Hence, a certain degree of compromise concerning the density in the arrangement of circular dimples has been necessary.
It is in such a context that, to arrange the dimples on a golf ball uniformly and to a high density, from two to ten types of dimples of differing diameter are disposed in such a way as to give the spherical surface of the ball the appearance of a regular octahedron or a regular icosahedron.
However, so long as circular dimples are used, the practical upper limit in the dimple surface coverage, defined as the ratio of the sum of the individual dimple surface areas to the total surface area of the spherical surface, is about 75% (that is, the surface coverage represented by the composite surface area of the lands is about 25%).
U.S. Pat. No. 6,290,615 describes a golf ball in which the land surface area has been reduced by providing, unlike the aforementioned dimples, projections (lattice members) that extend in a lattice over the smooth spherical surface and divide the surface into small hexagonal regions.
However, the small hexagonal regions demarcated by the lattice members are spherical surfaces having centers which coincide with the center of the ball and, because they are not dimples, are disadvantageous in terms of their air resistance-lowering effect.
In this connection, the present applicant has disclosed in U.S. Pat. No. 7,018,309 (corresponding Japanese application: JP-A 2005-319292) a golf ball having formed thereon, between mutually neighboring non-circular dimples, numerous narrow-width edge elements that serve as portions of the dimple edges, thereby increasing the dimple coverage on the surface of the ball and enhancing the air resistance-lowering effect.
However, in these existing golf balls, the presence of acutely or obtusely angled corners in the non-circular dimples having, for example, a polygonal shape as seen from above increases frictional resistance with air, resulting in a less than satisfactory improvement in the distance traveled by the ball.
Moreover, the present applicant has disclosed the dimples described in JP-A 2006-95281. Yet, in addition to an increase in the distance of travel, there is also a need for good moldability and practical improvements in the dimples.